Removal of silicon compounds from water



Patented Mar. 28, 1950 REMOVAL OF SILICON COMPOUNDS FROM WATER WilliamC. Badman, Midland, Mich., asslgnor to The Dow Chemical Company,Midland, Micln, a corporation of Delaware No Drawing.

Application July 17, 1944,

Serial No. 545,389

9 Claims. 1

This invention concerns a method of treating water to-purif the same. Itparticularly concerns the removal of dissolved silicon compounds fromwater.

It is known that water may be softened by passage through a bed of acation exchange agent and that it may be freed of most dissolved salts,alkalies, or acids by passage through a series of beds, at least one ofwhich is a bed of a cation exchange agent and another a bed of an anionexchange agent. However, natural waters, e. g. the waters of rivers,lakes, or wells, usually contain a small, but significant, amount ofdissolved silica which presumably is in the form of silicic acid or ametal silicate. Such direct treatment of the water with ion exchangeagents does not result in satisfactory removal of the dissolved silica.

For certain purposes, e. g. for use in highpressure boilers, it isimportant that the dissolved silicon compounds be removed as completelyas possible.

I have now found that the soluble silicon compounds often present inwater may be removed readily and nearly completely by treating the waterwith sufllcient hydrogen fluoride to convert the silicon compounds tofluosilicic acid and passing the water over an anion exchange agentwhich absorbs the fluosilicic acid and removes it from the water. I havealso found that such operations of removing dissolved silicon compoundsfrom water may advantageously be combined with operations for removingother dissolved ions, particularly cations, from the water to effectsubstantially complete purification of the latter. I also havediscovered satisfactory procedures for regenerating the anion exchangeagent to free the same of the absorbed silicon compounds and conditionit for re-employment in the process.

In order effectively to remove dissolved silicon compounds from water inaccordance with the invention, it is merely necessary to introduce intothe water sufficient hydrofluoric acid to convert pose are well known tothe art. In most instances such anion exchange agents arewater-insoluble organic resins containing amino groups, but certainother anion exchange agents which are satisfactory for the purpose arealso known. Examples of suitable anion exchange agents are resinousmeta-phenylenediamine and formaldehyde condensation products; resinouscondensation products of phenol, formaldehyde andpoly-alkylenepolyamines; and resinous condensation products of aliphaticamines, aldehydes and ketones; etc.

It is important that the water, containing the fluosilicic acid, besomewhat acidic at the time when it is fed into the bed of the anionexchange agent, not merely in order to avoid conversion of thefiuosilicic acid into a salt thereof, since such salt is sometimesformed together with the acid under acidic conditions, but because themetal salts of fiuosilicic acid tend to decompose into metal fluoridesand silicates under alkaline conditions. If this were to occur,satisfactory removal of the silicate by absorption on the anion exchangewould not be accomplished, i. e. silicic acid and its salts are notreadily absorbed by anion exchange agents. Peculiar-1y, although theanion-exchange agents are basic in the sense of forming salts withacids, they do not cause such decomposition, but, instead, react withthe fluosilicic acid to form a stable salt of the latter.

Such steps for converting dissolved silicon compounds into fluosilicicacid and for removing the latter from the water by absorption on ananion exchange agent may be accomplished, even though the water alsocontains other acids, e. g. hydrochloric or sulphuric acid. or containssoluble salts such as sodium chloride, sodium bromide, potassiumchloride, sodium sulphate, magnesium sulphate, etc., but the presence ofsuch other impurities, particularly'the salts, is sometimesdisadvantageous. Acids other than the fiuosilicic acid, if present, areabsorbed together with the fiuosilicic acid by the anion exchange agentand thereby reduce somewhat the absorptive capacity of the latter forthe fluosilicic acid, but they do not otherwise interfere with theprocess. However, dissolved salts often react with the fluosilicic acidto form another acid, e. g. hydrochloric acid, and a fiuosilicate, withestablishment of an equilibrium between such reactants and reactionproducts. In some instances, the anion exchange agent may absorb theacid other than fiuosilicic acid as readily, or more readily, than thelatter, in which case absorption and removal from the water of thefluosilicic acid may be incomplete. For this reason, dissolved salts arepreferably removed from the water prior to passage of the latter overthe anion exchange agent. Such removal of salts from the water isconvl'niently accomplished by passing the water over an acidic cationexchange agent, e. g. the acid form of a zeolite or a water-insolublesulphonatcd" or carboxylated organic resin, which absorbs metal cationsdissolved in the water and replaces them with hydrogen ions.

The step of passing the water through a cation exchange agent to removedissolved metal cations therefrom may advantageously be combined withthat of treating the water with hydrogen fluoride. Thus, by firsttreating the water with a water-soluble fluoride, such as sodiumfluoride, potassium fluoride, or ammonium bifluoride, etc., and passingit through a bed of a cation exchange agent which initially containsfree acid radicals, e. g. sulphonic acid or carboxylic acid radicals,the ammonium or metal cations dissolved in the water are observed by thecation exchange agent with resultant conversion of the fluoride intohydrogen fluoride. The latter reacts with the dissolved siliconcompounds to form fluosilicic acid. By carrying out the first step ofthe process in this particular manner, a salt of hydrofluoric acid,instead of the acid itself, may be used as a starting material andconsiderable economy is effected. Also, any soluble salts initiallypresent in the water are removed and efficient removal of thefluosilicic acid from the water in a subsequent step of the process isrendered more certain. After such passage over a cation exchange agent,the water may be passed over a basic anion exchange agent to effectabsorption and removal of the fluosilicic acid and at the same time toneutralize the water and render it substantially pure and suitable foruse in high-pressure boilers or for other purposes.

As procedure alternative to that just described, the basic anionexchange agent may initially be treated with hydrofluoric acid to form ahydrofluoride of the same, after which the water from which solublesilicon compounds are to be removed may be passed over the agent.Because of the fact that natural waters, after passage over an acidiccation exchange agent, usually contain, in addition to the dissolvedsilica, small amounts of acids such as sulphuric acid, hydrochloricacid, etc., it usually is advisable to convert only a portion, e. g.from to per cent, of the basic anion exchange agent to itshydrofluoride. During passage over the mixture of the basic exchangeagent and its hydrofluoride, the agent absorbs the soluble siliconcompounds and free acids from the water and is itself converted in partto a fluosilicate and in part to salts of the other acids absorbed fromthe water. The treatment of the basic anion exchange agent withhydrofluoric acid to form its hydrofluoride may be accomplished directlyor in other ways. For instance, an aqueous solution of ammoniumbifiuoride, or a metal fluoride, may be passed successively through abed of the acidic cation exchange agent and the basic anion exchangeagent so as to convert the latter partially into the form of itshydrofluoride. Thereafter, the water to be purified may be passed inseries through the same beds to remove soluble silicon compounds andother salts from the water. Such operations for initially converting theanion exchange agent into its hydrofluoride are advantageous in thatthey avoid the necessity of gradually introducing the hydrofluoric acidor soluble fluoride into the relatively large body of water which is tobe freed of silicon compounds.

During use in the process, the anion exchange agent is converted atleast in part to its fluosilicate and the cation exchange agent, whenemployed, is converted to. a salt. Periodically, it is necessary thateach such agent be regenerated in order to permit continued use of thesame. Methods for regenerating cation exchange agents are well known anddo not require detailed description. The passage of an aqueous solutionof an acid, such as hydrochloric or sulphuric acid, over such agent issuflicient to regenerate the same. The anion exchange agent maysimilarly be regenerated by treating the same with an aqueous solutionof an alkali such as sodium hydroxide, potassium hydroxide, or sodiumearbonate, etc. However, such direct treatment with an alkali usuallyresults in only partial and incomplete regeneration of the anionexchange agent, i. e. the soluble fluosilicate which is formedapparently decomposes with formation of a silicate that is retained to aconsiderable extent in the resin. As a result, the capacity of theregenerated anion exchange agent for the further absorption offluosilicic acid is considerably lower than that which it initiallypossessed. I have found that the regeneration may be accomplished morecompletely by treating the anion exchange agent with an aqueous acid, e.g. hydrochloric or suphuric acid, to displace fluosilicic acid from theanion exchange agent and convert the latter into a salt of the acid usedto effect the displacement. If desired, the resultant salt of the anionexchange agent, e. g. the sulphate or hydrochloride of said agent, maybe used directly for the further absorption of fluosilicic acid fromwater. However, such practice results in the introduction of anotheracid, e. g. sulphuric or hydrochloric acid, into the water and thisoften is undesirable. Accordingly, after treating the spent anionexchange agent with an acid to remove fluosilicate radicals therefromand form a different salt of the agent, the latter is preferably treatedwith a watersoluble basic compound, e. g. ammonia, sodium hydroxide, orsodium carbonate, etc., to decompose the salt and reconvert the anionexchange agent into its basic form. The agent is then in condition forre-employment 0 in the process for the treatment of water to removedissolved silicon compounds from the latter.

The following example describes one way in which the principle of theinvention has been applied, but is not to be construed as limiting theinvention:

Example such as sodium chloride, calcium chloride, etc.,

was passed first through a bed of an acidic, granular, alkali-insolublesulphonated phenolformaldehyde resin, which served as a cation exchangeagent, and then through a bed of the above-mentioned anion exchangeagent. The

water which flowed from the second bed, i. e. the bed of the anionexchange agent. was, in the early stages of the operation, substantiallypure and free of silicon compounds. However, the passage of river waterthrough the two beds was continued until the water flowing from thesecond bed contained silicon compounds in amount correspondingapproximately to the initial concentration of such compounds in theriver water. In this first stage of the process no attempt was made todetermine the amount of water that could be freed of silicon compounds,since it was the absorptive capacity of the anion exchange agent afterbeing generated, rather than when first used, which was of greatestinterest. After the anion exchange agent had absorbed its capacity ofsilicon compounds, the flow of river water was discontinued and anaqueous hydrochloric acid solution of per cent concentration was passedthrough the two beds. The cation exchange agent was thereby regeneratedand the anion exchange agent was rendered nearly free of siliconcompounds and converted to its hydrochloride. Regeneration of the anionexchange agent was completed by passing over the same an aqueousammonium hydroxide solution of 4 per cent concentration. The anionexchange agent was again converted in part to its hydrofluoride afterwhich the passage of river water through the two beds of the ionexchange agents was continued. After flushing the two beds free of theliquors used in regenerating the same,

there was obtained as the eilluent from the second bed, i. e. the bed ofthe anion exchange agent, purified water containing silicon compounds inamount corresponding to less than 0.2 part of silica and containingotherionizable salts which, when calculated as being sodium chloride,corresponded to less than 0.2 part of sodium chloride, per million partsof the water. Water of this purity was obtained in amount correspondingto 1,520 gallons of water per cubic foot of the bed of the anionexchange agent before the contents of dissolved silica and other anionsin the water flowing from the bed increased quite sharply to approachthose of the untreated river water. The amount of dissolved siliconcompounds absorbed from the water by the anion exchange agentcorresponded to 0.0454 pound of silica per cubic foot of the bed of saidagent. When the water flowing from the system had a silicon contentcorresponding approximately to that of the untreated water, the flow ofriver water was interrupted and an aqueous hydrochloric acid solution of5 per cent concentration was passed through the bed of the anionexchange agent. The liquor which flowed from the bed during the latteroperation was analyzed and found to contain silicon compounds in amountcorresponding to 0.0444 pound of silica per cubic foot of the bed of theanion exchange agent. This corresponded to displacement, from the anionexchange agent, of approximately 98 per cent of the silicon compoundswhich had been absorbed by the agent.

Other modes of applying the principle of the invention may be employedinstead of those described, change being made as regards the methodherein disclosed, provided the step or steps stated by any of thefollowing claims or the equivalent of such stated step or steps beemployed.

I therefore particularly point out and. distinctly claim as myinvention:

1. In a method of purifying water which contains dissolved cations and anaturally occurring dissolved silicon compound, the steps of passing thewater through a bed of a substantially waterinsoluble acidic cationexchange agent and thereafter through a bed comprising a hydrofluorideof a substantially water-insoluble, anion exchange agen 2. In a methodof purifying water which contains a naturally occurring dissolvedsilicon compound, the steps of treating the water with a water-solubleand an ionizable salt of hydrofluoric acid and passing the resultantsolution over, and in contact with, a bed of a substantiallywater-insoluble acidic cation exchange agent and thereafter over, and incontact with, a bed of a substantially water-insoluble anion exchangeagent in its basic form.

3. In a method of freeing water of a dissolved silicon compound, thesteps of treating water, which contains a dissolved silicon compound butwhich is substantially free of metal ions, with both a substantiallywater-insoluble hydrofluoride of an anion exchange agent and asubstantially water-insoluble anion exchange agent in its basic form.

4. A process of purifying water which comprises adding to watercontaining silica and other ionizable solids a quantity of an alkalimetal fluoride sufficient to convert substantially all of the silicapresent to fluosilicic acid, and passing the so-treated water through atleast one series of beds of ion active materials comprising, first, acation exchange material active for the removal of cations from waterand, second, an anion exchange material active for the removal of anionsfrom water.

5. In a method of freeing water of dissolved silicon compounds, the stepof passing the water through a bed of a substantially water-insolubleanion exchange material comprising a hydrofiuoride of an anion exchangeagent.

6. In a method of freeing water of dissolved silicon compounds, thesteps of treating a substantially water-insolublebasic anion exchangeagent with hydrogen fluoride in amount insufficient to convert all ofsaid agent to a hydrofluoride thereof, and thereafter passing the water,containing dissolved silicon compounds, through a bed of thethus-treated anion exchange material.

7. A process of purifying an aqueous fluid which comprises adding to anaqueous fluid containing silica and other ionizable solids a quantity ofan alkali metal fluoride suflicient to convert substantially all of thesilica present to fluosilicic acid, and passing the so-treated aqueousfluid through at least one series of beds of ion active materialscomprising, first, a cation exchange material active for the removal ofcations from water and, second, an anion exchange material active forthe removal of anions from water.

8. A process of purifying an aqueous fluid which comprises adding to anaqueous fluid containing silica and other ionizable solids a quantity ofsodium fluoride suiflcient to convert substantially all of the silicapresent to fluosilicic acid, and passing the so-treated aqueous fluidthrough at least one series of beds of ion active materials comprising,first, a cation exchange material active for the removal of cations fromwater and, second, an anion exchange materiai active for the removal ofanions from water.

9. In a method of purifying water which contains a dissolved ionizablesalt and also a naturally-occurring dissolved silicon compound, the

steps of passing the water through at least one series of beds of ionactive materials comprising, first a substantially water-insolubleacidic cation exchange material and, second, a substantiallywater-insoluble bed of both. an anion exchange material in its basicform and a hydrofluoride of said anion exchange material.

WILLIAM C. BAUMAN.

REFERENCES CITED Number 8 UNITED STATES PATENTS Name Date Doremus May28, 1889 Wixford Dec. 12, 1905 Green June 1, 1937 Goetz Dec. 6, 1938Liebknecht Apr. 18, 1939 Urbain et a1 May 9, 1939 Urbain et a1 Aug. 13,1940 Behrman Jan. 7, 1941 Holmes et a1 Apr. 8, 1992 Certificate ofCorrection Patent N0. 2,502,120 March 28, 1950 I WILLIAM C. BAUMAN It ishereby certified that errors appear in the printed specification of theabove numbered patent requiring correction as follows:

Column 3, line 20, for the Word observed read absorbed; column 5, line14, for generated read regenerated;

and that the said Letters Patent should be read with these correctionstherein that the same may conform to the record of the case in thePatent Oifice.

Signed and sealed this 11th day of July, A. D. 1950.

THOMAS F. 'MURPHY,

Assistant Oommissz'oner of Patents.

Lem A

4. A PROCESS OF PURIFYING WATER WHICH COMPRISES ADDING TO WATERCONTAINING SILICA AND OTHER IONIZABLE SOLIDS A QUANTITY OF AN ALKALIMETAL FLUORIDE SUFFICIENT TO CONVERT SUBSTANTIALLY ALL OF THE SILICAPRESENT TO FLUOSILICIC ACID, AND PASSING THE SOL-TREATED WATER THROUGHAT LEAST ONE SERIES OF BEDS OF ION ACTIVE MATERIALS COMPRISING, FIRST, ACATION EXCHANGE MATERIAL ACTIVE FOR THE REMOVAL OF CATIONS FROM WATERAND, SECOND, AN ANION EXCHANGE MATERIAL ACTIVE FOR THE REMOVAL OF ANIONSFROM WATER.
 5. IN A METHOD OF FREEING WATER OF DISSOLVED SILICONCOMPOUNDS, THE STEP OF PASSING THE WATER THROUGH A BED OF ASUBSTANTIALLY WATER-INSOLUBLE ANION EXCHANGE MATERIAL COMPRISING AHYDROFLUORIDE OF AN ANION EXCHANGE AGENT.